Solid anti-friction devices – materials therefor – lubricant or se – Lubricants or separants for moving solid surfaces and... – Organic phosphorus compound – wherein the phosphorus is...
Reexamination Certificate
1999-06-30
2001-01-09
McAvoy, Ellen M. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Lubricants or separants for moving solid surfaces and...
Organic phosphorus compound, wherein the phosphorus is...
C508S441000, C508S545000, C508S584000
Reexamination Certificate
active
06172014
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of reducing compressor gas leakage by providing a compression cylinder with a lubricant comprising less than about 1% of a synergistic mixture of antioxidants.
BACKGROUND ART
The escalating requirements for lubricants used in compression cylinders associated with the dynamic sealing of ethylene within the compressor cylinder and the hydraulic fatigue of pressure containing components subjected to cyclic pressures is considered one of the most demanding aspects of high-pressure manufacture of polyethylene. These aspects are discussed in more detail in
Lubrication Engineering
, Volume 37, 4, 203-208 (1981). High demands for large scale production require increasingly improved lubricants. This objective becomes particularly difficult to achieve given the sophisticated and capital-intensive nature of the process and challenges the limitations of polyethylene manufacture. Thus, the combined requirements of high-pressure equipment and cylinder life pose a challenge which, to date, has not been satisfactorily achieved.
U.S. Pat. No. 4,654,154 to Wilkelski discloses a conventional method of reducing cylinder gas leakage from compressors used in high pressure olefin polymerization processes wherein from about 3 to about 10 wt % of an antioxidant is added to the lubricating fluid provided to a compressor cylinder. Wilkelski discloses that the problem of increased gas leakage is due to an oxidation reaction occurring in the narrow annular passageway between the plunger and the cylinder packing.
The oxidation can take place due to the presence of unreacted peroxide initiators in the recycled ethylene and/or the introduction of a polymerization initiator, that is, oxygen gas to the ethylene before compression. The unwanted oxidation leads to undesirable consequences including buildup of excessive heat, high pressures and resultant high mechanical stress.
Despite the benefits achieved by the Wilkelski method, the provision of the additional antioxidant forces the cost of practicing the method upwards, thereby adversely affecting the commercial viability of the process.
There exists a need to provide an efficient and cost-effective technique to satisfy the demands of high-pressure polyolefin manufacture.
DISCLOSURE OF THE INVENTION
An object of the present invention is a method of reducing compressor gas leakage comprising the step of: providing a compression cylinder with a lubricant comprising less than about 1.0 wt % of an antioxidant.
Additional objects and advantages of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other objects are achieved in part by a composition comprising a lubricant, and at least two antioxidants, wherein the antioxidants are present in an amount less than about 1 wt %, based on the total weight of the composition, and wherein the composition reduces cylinder gas leakage from a compressor used in high pressure olefin polymerization and increases time intervals between replacement of worn cylinder assemblies.
Additional objects and advantages of the present invention will become readily apparent to those having ordinary skill in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.
DESCRIPTION OF THE INVENTION
The present invention addresses and solves problems stemming from the use of unnecessarily high amounts of costly antioxidants in connection with lubricating compression cylinders used in high pressure olefin polymerization processes.
In particular, the present invention is based, in part, on the surprising and unexpected discovery that equal or improved lubricant stability may be achieved at much lower levels of antioxidant through the use of synergistic mixtures of antioxidants comprising different blends of phosphite antioxidants, hindered phenols, and secondary amines. Numerous practical advantages in handling the smaller volume of solids stem from the present invention, and include less costly equipment for handling and less time required for batch preparation.
The lubricant component of the present invention may be any commercially available compound known to perform as a cylinder lubricant in a high pressure compressor for providing olefin feed to a polymerization zone. Examples include mineral oils and especially white mineral oils of 250 to 1200 SUS viscosity. The white mineral oils may also be combined with a polymeric thickener such as polybutene to adjust the viscosity to the desired level.
The antioxidant component of the present invention may be any antioxidants known to those of ordinary skill in the art. Examples include antioxidants comprising phosphite compounds and aminic and phenolic antioxidants.
An antioxidant of the present invention may be a phosphite antioxidant, for example, a phosphite antioxidant selected from the group consisting of trinonylphenyl phosphite, tris-tert-butylphenyl phosphite, tridecylphosphite, triphenylphosphite, trioctylphosphite, alkylphenylphosphites, and dilaurylphosphite. Preferred phosphite antioxidants include tris(2,4-di-tert-alkylphenyl)phosphite, more preferably tris(2,4-di-tert-butylphenyl)phosphite. The term “alkyl” is used throughout the present specification to mean an unbranched or a branched alkyl chain having from 1 to 8 carbon atoms. The amount of phosphite antioxidant characteristically present in the composition are critical high oxidation induction values and the total amount of antioxidant present does not exceed about 1 wt %, based on the total weight of the composition. A preferred tris(2,4-di-tert-butylphenyl)phosphite is Irgafos 168 sold commercially by Ciba-Geigy under that name.
Another example of an oxidant which may be useful in the present invention is a phenolic antioxidant. Preferred phenolic antioxidants are hindered phenols such as thiodiethylene bis(3,5-di-tert-alkyl-4-hydroxyhydrocinnamates, more preferably thiodiethylene bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate which is sold commercially under the name Irganox 1035 by Ciba-Geigy, and tetrakis[methylene(3,5-di-tert-alkyl-4-hydroxyhydrocinnamate)]methanes, more preferably tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane is sold commercially under the name Irganox 1010 by Ciba-Geigy. Another example of a hindered phenol useful in the present invention is butylated hydroxytoluene (BHT). The amount of phenolic antioxidant characteristically present in the composition is not particularly critical so long as the total amount of antioxidant present does not exceed about 1 wt %, based on the total weight of the composition.
A further example of an oxidant which may be useful in the present invention is an aminic antioxidant. Preferred aminic antioxidants include secondary aromatic amines such as diarylamines, e.g., diphenylamine, and modifieddiarylamines, e.g., N-phenyl-g-naphthylamine, p-isopropoxydiphenylamine, mono and dioctyldiphenylamine, bis-diarylamines and modified bisdiarylamines, such as N,N-diphenyl-p-phenyldiamine. The amount of aminic antioxidant characteristically present in the composition is not particularly critical so long as the total amount of antioxidant present does not exceed about 1 wt %, based on the total weight of the composition.
An especially preferred antio
McAvoy Ellen M.
McDermott & Will & Emery
Pennzoil-Quaker State
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